Removal of conductive coating from dimensionally stable electrodes

ABSTRACT

A MOLTEN SALT BATH CONSISTING OF AN ALKALI METAL HYDROXIDE AND AN ALKALI METAL SALT OF AN OX IDIZING AGENT IS USED TO COMPLETELY REMOVE THE CONDUCTIVE COATING FROM A DIMENSIONALLY STABLE ELECTRODE.

REMOVAL OF CONDUCTIVE COATING FROM DIMENSIONALLY STABLE ELECTRODESCharles F. Hitzel, Painesville, Ohio, assignor to Diamond ShamrockCorporation, Cleveland, Ohio No Drawing. Filed Feb. 24, 1969, Ser. No.806,343 Int. Cl. C22b 11/00; C23g N28 US. Cl. 134-2 8 Claims ABSTRACT OFTHE DISCLOSURE A molten salt bath consisting of an alkali metalhydroxide and an alkali metal salt of an oxidizing agent is used tocompletely remove the conductive coating from a dimensionally stableelectrode.

BACKGROUND OF THE INVENTION Recent years have seen the development ofthe use of dimensionally stable electrodes, particularly dimensionallystable anodes for use in the electrolysis of alkali metal halidesolutions in both mercury and diaphragm-type electrolytic cells. Inaddition to their voltage advantage over the graphite anodes which theyare replacing, these dimensionally stable anodes, because of theirresistance to the cell environment, afford considerable economic andmaintenance advantages. While graphite anodes decompose relativelyrapidly under operating cell conditions, thereby requiring frequentadjustment of the anode-cathode gap as well as periodic replacement ofthe anode itself, the dimensionally stable anodes, by definition,undergo no significant physical change, even during extended periods ofoperation, thus reducing down-time of the cell and maintenance andreplacement requirements. Despite their extremely low wear rate,however, even dimensionally stable anodes eventually require attentionsince the conductive coating on the surface of the anodes slowly becomespassivated or depleted resulting in an increase in the voltage at whichchlorine is discharged at the anode surface and consequent uneconomicoperation. When this occurs, it is necessary to remove the dimensionallystable anodes from the cells and replace them with anodes having activeconductive surfaces.

For the most part these dimensionally stable electrodes consist of aconductive coating upon a titanium or tantalum substrate. Obviously, inview of the high cost of the substrate metal, as well as the cost offabricating the metal into the required shape for use in electrolyticapplications, it is not practical to discard a passivated or wornelectrode structure. Thus, it would be desirable to clean the surface ofthe dimensionally stable electrode in order that a new conductivecoating could be applied thereon and the entire structure re-used.

Unfortunately, however, while an uncoated titanium or tantalum structureis not difiicult to clean, it is found that, even when an electrode hasfailed via passivation, a portion of the electrically conductive coatingstill remains upon the surface of the substrate. These conductivecoatings, which often contain a portion of noble metal or otherrelatively inert materials, are very diflicnlt to remove by ordinarychemical or electro-chemical means. Nevertheless the coatings must beremoved if a clean, uniform surface is to be presented for subsequentdeposition of an adherent conductive coating.

STATEMENT OF THE INVENTION It is an object of the present invention toprovide a method for removing the conductive coating from adimensionally stable electrode.

Another object of the present invention is to provide a United StatesPatent ice method for removing the electrically conductive coating froma dimensionally stable electrode in such a manner that the valuableconstituents of said coating may be recovered.

A further object of the present invention is to provide a method for theremoval of the conductive coating from a dimensionally stable electrode,which method results in a clean structure on which a new conductivecoating may be applied with excellent adherence.

A still further object of the invention is to provide a method whichrapidly removes the conductive coating from a dimensionally stableelectrode with a minimum of damage to the electrode substrate.

These and further objects and advantages of the present invention willbecome apparent to those skiled in the art from the specification andclaims which follow.

It has now been found that the conductive coating may be rapidly andefliciently removed from a dimensionally stable electrode, withoutsubstantial detrimental effect to the electrode structure itself, bycontacting the dimensionally stable electrode with a molten salt bathconsisting of a mixture of from 1 to 15 parts by weight of an alkalimetal hydroxide and one part by weight of an alkali metal salt of anoxidizing agent. Upon removal from contact with the molten salt bath,cooling and rinsing any adherent fused salt from the electrodestructure, said structure is suitable for etching and application of anew electrically conductive coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS By the use of the term alkalimetal hydroxide in the specification and claims it is intended to referto the hy droxides of sodium, potassium and lithium or mixtures thereof,notably sodium and potassium hydroxide and preferably potassiumhydroxide. As is mentioned hereinabove this alkali metal hydroxide willbe present within the range of l to 15 parts of alkali metal hydroxideper 1 part of oxidizing agent, all parts by weight. At this time thepreferred range is from 3:1 to 10:1. A typical and especially preferredbath consists of 5 parts of potassium hydroxide and 1 part of an alkalimetal salt of an oxidizing agent.

It has been found by experience that at least equal quantities ofhydroxide and oxidizing agent are required to obtain satisfactoryresults. Preferably a large excess of hydroxide to oxidizing agent isused. When approaching the 1:1 ratio, from an excess of hydroxide, it isfound that the amount of time required to effect complete stripping ofthe coating increases greatly, all other factors being equal.Conversely, when approaching the upper limits of the 15:1 ratio, nosacrifice in stripping rate is noticed, although replenishment of theoxidizing agent is more frequently required to maintain the minimumconcentration.

By the use of the phrase alkali metal salt of an oxidizing agent isintended to refer to the sodium, potassium and lithium salts of suchagents. Again, sodium and potassium salts are preferred, with potassiumbeing especially preferred at this time. The oxidizing portion, oranion, of the salt is selected from the group consisting of nitrates,chlorates, peroxides, permanganates and perchlorates. Examples ofsuitable oxidizing agents are potassium nitrate, sodium nitrate, sodiumchlorate, potassium perchlorate, potassium permanganate, sodiumperoxide, and the like. Obviously, mixtures of oxidizing agents may beused in many instances. Especially preferred at this time are sodium andpotassium nitrate.

An advantage of the present invention, which requires that at least a1:1 ratio of ingredients be used, and preferably a large excess ofhydroxide, is that the cost of the hydroxide is far less than that ofthe oxidizing agent.

3 That is, a bath requiring a reverse ratio of ingredients would besignificantly more expensive to establish and maintain.

The term dimensionally stable electrodes is used throughout thespecification and claims and is intended to refer primarily to titaniumor tantalum structures carrying an electrically conductive coating. Inaddition to pure titanium and tantalum, alloys of these metals are alsouseful, as are other conductive metals, such as copper and aluminum,clad or coated with titanium, tantalum or alloys thereof. Other valvemetals not adversely affected by the molten salt bath may also be usedin special applications. These electrode structures are generally in theform of a solid sheet, a perforated or slotted sheet, or an expandedmesh configuration, although the shape of the electrode has no bearingupon the practice of the present invention.

It has been found, somewhat surprisingly, that the identity of theelectrically conductive coating is likewise not critical to the practiceof the present invention, that is, a variety of electrically conductivecoatings may be removed by this technique. Thus, noble metal coatingssuch as platinum, palladium, iridium and ruthenium metals and alloysthereof are rapidly and readily removed according to the invention.Likewise the oxide, or ceramic, coatings are also effectively andefliciently removed. These latter coatings include, in addition to theoxides of the noble metals themselves, mixtures of oxides of noblemetals with oxides of valve metals (e.g., titanium, tantalum, aluminum);mixtures of oxides of noble metals, valve metals and other metals; aswell as mixtures of certain of the foregoing oxides with noble metals inthe metallic form. Examples of such coatings include: ruthenium oxide;ruthenium oxide-titanium oxide; ruthenium oxide-titanium oxide-aluminumoxide; ruthenium oxide-titanium oxide-iridium; ruthenium oxidetitaniumoxide-tantalum oxide, and others.

In order to effectively and completely remove the elec tricallyconductive coating from the titanium or tantalum base, it is onlynecessary to contact the coated structure with the molten salt bath fora period of time dependent upon the thickness of the coating to beremoved and the temperature of the salt bath. A Wide range oftemperatures, e.g., 350 C.500 C. are effective in this invention so longas the temperature chosen is sufficient to insure maintenance of amolten state but not so high as to result in ignition of the substratemetal. Generally it has been found that the most effective strippingoperation can be accomplished at a temperature within the range of 400C.450 C. Using a salt bath within this temperature range to remove thecoating from an average dimensionally stable electrode will require lessthan 15 minutes, generally less than 5 minutes and often as low as -1minute.

A significant advantage of the present invention lies in the fact that,despite the speed of the stripping operation and the fact that a varietyof coatings may be removed with equal ease and effectiveness,substantially no damage is incurred by the titanium or tantalumstructure itself. Thus, experience to date has shown that less than 5percent, and generally less than 2 percent, by weight of the metal islost from the structure itself in each case.

After the stripping operation is complete, the electrode is removed fromthe molten salt bath and, after cooling, the fused salt adhering to theelectrode structure may be readily removed by rinsing in water todissolve the salt. Obviously, the electrically conductive coatingstripped from the dimensionally stable electrode remains in the moltensalt bath and at periodic intervals the components of the coating may berecovered in a variety of ways, provided of course that they are of sucha nature as to be worth the cost of a recovery operation. An especiallypreferred manner of providing the salt bath in condition for economicalrecovery of the coating constituents involves the concentration of theseconstituents to a maximum level. This may be readily accomplished byadding fresh amounts of the hydroxide-oxidizing salt mixture, in theproper proportions, to the molten bath as it is depleted by us and bydrag-out. This serves to effect the concentration of the recoverablecomponents while still maintaining an effective and efficient strippingoperation. When the saturation point of the bath is reached, withrespect to its ability to accept more stripped coating without asacrifice in stripping efficiency, recovery may be attempted. Forexample, the components of the coating, now in a highly oxidized state,may be recovered by dissolving the fused salt melt and precipitating andfiltering the materials therefrom. Alternately, recovery can be byelectrolytic means.

A problem of major concern when attempting to recoat electrodestructures after stripping of the old coating by prior methods, has beenthe difficulty of obtaining any degree of adherence of the subsequentlyapplied coating to the cleaned electrode structure. According to thepresent invention however, not only is the degree of adherence obtainedat least equal to that of a new electrode, but the degree of etchingrequired prior to the application of the coating is significantlyreduced.

In order that those skilled in the art may more readily understand thepresent invention and certain preferred embodiments by which it may becarried into effect, the following specific examples are afforded.

EXAMPLE 1 An anode comprising an expanded titanium mesh substrate havingdeposited thereon an electrically conductive coating of rutheniumdioxide-titanium dioxide, is first cleaned of extraneous material suchas cell putty, salt and the like and thereafter heated to dryness. Thestructure is then immersed in a clear salt bath containing 5 parts byweight of KOH and 1 part by weight of KNO and maintained at atemperature of 450 C. (1-20" C.). A nickel tank is used to preventcorrosion. After 5 minutes in the molten salt bath, the anode structureis carefully removed, the still molten salt is permitted to drain backinto the melt, and the anode is allowed to cool, at which time anyadhering fused salt is removed by rinsing in Water. Upon examination,the surface of the titanium is seen to be bright, smooth and uniform.

The thus-cleaned structure is then etched prior to the application of a.fresh coating of electrically conductive material and it is found thatonly a 10 minute etch in boiling, azeotropic, hydrochloric acid isrequired, compared with the normal 45-60 minute etch used when initiallypreparing the titanium for coating.

After recoating the titanium structure with a RuO TiO coating mixture,it is found that an electrode having voltage characteristics and a wearrate at least comparable to the original electrode is obtained.

EXAMPLE 2.

An expanded titanium mesh anode having a platinum metal coating thereonis cleaned and dried as in Example 1. In this instance the molten saltbath is a mixture of 5 parts of sodium hydroxide and 1 part of sodiumnitrate maintained at a temperature of about 350 C. After 5 minutes, thestructure is removed from the melt, cooled and rinsed with water asbefore, at which time it is noticed that the surface of the mesh isperfectly clean with the exception of a few scattered white spots. Theweight loss on the substrate during stripping is determined to be 2.1percent. During the subsequent hydro chloric acid etching operation thewhite spots disappear and the cleaned, etched structure is totallyacceptable for recoating with an electrically conductive material.

Although the invention has been described with reference to certainspecific embodiments thereof, it is not to be so limited since changesand alterations may be made therein which are within the full andintended scope of the appended claims.

I claim:

1. A method for removing the electrically conductive coating from adimensionally stable titanium electrode structure which method comprisescontacting said structure with a molten salt bath consisting of amixture of from 1 to 15 parts by weight of an alkali metal hydroxide and1 part by weight of an alkali metal salt of an oxidizing agent, for aperiod of time sufficient to effect removal of said coating.

2. A method as in claim 1 wherein the alkali metal hydroxide is selectedfrom the group consisting of potassium hydroxide and sodium hydroxideand the alkali metal salt of the oxidizing agent is selected from thegroup consisting of sodium and potassium salts of nitrates, chlorates,perchlorates, permanganates and peroxides.

3. -A method as in claim 1 wherein the alkali metal hydroxide ispotassium hydroxide and the alkali metal salt of the oxidizing agent ispotassium nitrate.

4. A method as in claim 1 wherein the ratio of alkali metal hydroxide toalkali metal salt of an oxidizing agent is within the range from 3:1 to:1.

5. A method as in claim 1 wherein the ratio of alkali metal hydroxide toalkali metal salt of an oxidizing agent is 5:1.

6. A method as in claim 1 wherein the molten salt bath is maintained ata temperature within the range of 400 to 450 C.

7. A method for removing the electrically conductive coating from adimensionally stable titanium electrode structure by stripping saidcoating from said electrode structure in a molten salt bath containingfrom 1 to 15 parts by weight of an alkali metal hydroxide and 1 part byWeight of an alkali metal salt of an oxidizing agent and recovering thethus-stripped coating, which method includes the step of increasing theconcentration of the stripped coating in the molten salt bath byperiodic additions of the hydroxide and oxidizing agent as the bathbecomes depleted in these components.

8. A method for removing the electrically conductive coating from adimensionally stable titanium electrode structure by stripping saidcoating from said electrode structure in a molten salt bath containingfrom 1 to 15 parts by weight of an alkali metal hydroxide and 1 part byweight of an alkali metal salt of an oxidizing agent, which methodincludes the step of recovering the stripped coating from the salt bath.

References Cited UNITED STATES PATENTS 2,738,293 3/1956 Spence 252-103 X2,794,001 5/1957 Carter 134-2 X 2,891,881 6/1959 Jaffe l342 3,030,2394/1962 Mekjean l34-29 3,502,503 3/1970 iBartlo l34-29 X 3,573,100 3/1971Beer l34--3 MORRIS O. WOLK, Primary Examiner D. G. MILLMA N, AssistantExaminer US. Cl. XJR. -101, 121

